Why do wet things look darker than dry things?

July 7, 1989

Dear Cecil:

This has bugged me all my life: why do wet things look darker than dry things?

— Kathleen H., Brookline, Massachusetts

Cecil replies:

We'll take this in stages, Kathleen. Stop me when you can't take any more. (1) Talk-Show-on-Commercial-Radio version: Because when something is wet, light bounces around inside it more (as opposed to merely bouncing off the surface) before being reflected back to the eye. The more the light bounces, the more of it gets absorbed, the less reaches the eye, and the darker the object appears.

This is fine for most purposes, but sometimes I have to escalate to (2) the Talk-Show-on-PBS version, which goes on to add that the reason the light bounces more is that the moisture increases the average scattering angle of the light particles. When the photons strike the surface of the wet material most of them bounce forward and hence deeper into the stuff rather than backward toward the eye.

At this point I'm sometimes tempted to launch into (3) the Ph.D.-thesis version, which comes complete with wavelengths, angstroms, and electron shells, but invariably the host's eyes start to glaze over and I find myself swiftly segueing into the latest on Rocky and Bullwinkle. Being the world's smartest human is all very well, but even I know when to quit.